Hydrogels are water-swellable or water-swollen materials whose structure is defined by a crosslinked network of hydrophilic homopolymers or copolymers. The hydrophilic homopolymers or copolymers can be water-soluble in free form, but in a hydrogel they are rendered insoluble due to the presence of covalent, ionic, or physical crosslinks. In the case of physical crosslinking, the linkages can take the form of entanglements, crystallites, or hydrogen-bonded structures. The crosslinks in a hydrogel provide structure and physical integrity to the network.
Hydrogels can be classified as amorphous, semicrystalline, hydrogen-bonded structures, supermolecular structures, or hydrocolloidal aggregates. Numerous parameters affect the physical properties of a hydrogel, including porosity, pore size, nature of gel polymer, molecular weight of gel polymer, and crosslinking density. The crosslinking density influences the hydrogel's macroscopic properties, such as the swelling ratio, compressive modulus, or mesh size. Pore size and shape, pore density, and other factors can impact the surface properties, optical properties, and mechanical properties of a hydrogel.
Hydrogels can attain a wide variety of mechanical properties. In general, however, hydrogels are observed to be pliable or rubbery, with a lubricious surface. Hydrogels are generally characterized by a low coefficient of friction owing to the water content and water-release properties at the surface. Frictional behaviors of hydrogels do not conform to Amonton's law, which states that the friction force is proportional to normal (i.e., orthogonal to the plane of motion) force. Unique load dependencies are observed for the friction coefficient of hydrogels: as load increases, friction coefficient decreases. As the hydrogel deforms under load, part of the water is squeezed out from the bulk gel and serves as a lubricant, leading to boundary lubrication or hydrodynamic lubrication.
Hydrogels have been fabricated from a variety of hydrophilic polymers and copolymers. Poly(vinyl alcohol), poly(ethylene glycol), poly(vinyl pyrrolidone), polyacrylamide, and poly(hydroxyethyl methacrylate), and copolymers of the foregoing, can be given as examples of polymers from which hydrogels have been made. Further, naturally occurring polymers such as chitosan, agarose, hyaluronic acid, gelatin, collagen or mixtures of these types of natural materials may be used with other synthetic polymers to create hydrogels
Hydrogels can be neutral or ionic, based on the type of charges of any pendant groups on the polymer chains. Hydrogels may exhibit swelling behavior that is dependent on and responsive to the external environment. Environmentally or physiologically responsive hydrogels, sometimes referred to as “intelligent” hydrogels, can exhibit drastic changes in swelling ratio due to changes in the external pH, temperature, ionic strength, nature of the swelling agent, and exposure to electromagnetic radiation. Hydrogels that exhibit pH-dependent swelling behavior generally contain either acidic or basic pendant groups. In aqueous media of appropriate pH and ionic strength, the pendant groups can ionize, resulting in fixed charges on the gel.
Over the past three to four decades, hydrogels have shown promise for biomedical and pharmaceutical applications, mainly due to their high water content and rubbery or pliable nature, which can mimic natural tissue. Biocompatible hydrogels can be engineered to be either degradable or resistant to degradation. An additional advantage of hydrogels, which has only recently been appreciated, is that they may provide desirable protection of drugs, peptides, and especially proteins from the potentially harsh environment in the vicinity of a release site. Thus, such hydrogels could be used as carriers for the delivery of proteins or peptides by a variety of means, including oral, rectal, or in situ placement. Transport of eluents either through or from a hydrogel is affected by pore size and shape, pore density, nature of polymer, degree of hydration, and other factors. Hydrogels can also act as transport barriers, due to a size exclusion phenomenon. Also relevant in drug delivery applications are pH and ionic strength sensitivity, as exhibited by hydrogels of some ionic or ionizable polymers.
Hydrogels have been used and proposed for a wide variety of biomedical and drug-delivery applications. For example, hydrogels have been utilized in controlled-release devices to achieve delivery of a drug or protein over time, and hydrogels have been widely employed in the fabrication of contact lenses. Hydrogels can be made to have properties similar to cartilage and are one of the most promising materials for meniscus and articular cartilage replacement. An overview of considerations for biological and medical applications of hydrogels can be found in Peppas, et al., Ann. Rev. Biomed. Eng. 2, 9 (2000), which is incorporated by reference in its entirety.
Poly(vinyl alcohol) (“PVA”) is a polymer that has been studied extensively for potential biomedical applications. PVA hydrogels can be produced, for example, from aqueous solution via repeated freezing and thawing cycles that increase the order of the crystals, thereby changing the dissolution properties, mesh size, and diffusion properties of the polymer. An overview of developments in PVA hydrogels can be found in Peppas, et al., Adv. Polymer Sci. 153, 37 (2000), which is incorporated by reference in its entirety.
One desirable feature of PVA hydrogels for biomedical applications is that the hydrogels are very absorbent. PVA hydrogels can have a moisture content of upwards of 70% in many cases. In contrast, polyurethane hydrogels commonly employed in implantable devices are generally characterized by low moisture content, on the order of a few percent.
Problems commonly associated with hydrogels that possess desirable absorbent properties include low mechanical strength and low shear strength. Devices made from PVA hydrogels have been observed to fail due to wear, such as by tearing, abrasion, or shredding. Thus, achieving improved mechanical strength and other physical properties for implants made from PVA hydrogels is desired.